Altmire and Critz, two Democrats who have been allies in the House, were pitted against each other by redistricting, which combined their districts and forced the two to square off in a primary. The redrawn district incorporates Johnstown, Critz's culturally conservative but economically liberal base, and the suburban Pittsburgh territory that is home to many of Altmire’s supporters.

"We're encouraged. The fact is Jason starts off representing two-thirds of the district, yet he's only able to get 50 percent of the votes," said Critz spokesman Mike Mikus. "That shows Mark has a lot of room to grow."

The poll of 503 likely primary voters was conducted Jan. 10-15 and had a margin of error of 4.4 percentage points.

Reports also emerged Friday that Mike Turzai, the majority leader in Pennsylvania’s state House, was also entering the race. The AP reported that Turzai has been telling top GOP leaders in the state of his plans.

Altmire responded quickly to those reports, calling it another reason why the Democratic primary was of utmost importance.

“Democrats need to nominate the candidate who can run the strongest campaign in the general election against Rep. Turzai, in the Republican-leaning district that he drew for himself,” Altmire said in a statement.

Critz also announced he would be unveiling a major endorsement on Monday at the headquarters of United Steelworkers in Pittsburgh. An endorsement by the influential union would be a major boon for Critz.

Unions have made no secret of their distaste for Altmire, and their support could be crucial to determining the race’s victor.

—This post was updated at 2:53 p.m.